1. Field of the Invention
The present invention relates generally to a braking system for a wheeled vehicle. More particularly, the present invention relates to a braking system which provides a function of communicating to wheel cylinders a fluid pressure which is a multiple of a master cylinder pressure, to assist the vehicle operator in applying the brakes (hereinafter power brake function) and a function of proportioning the braking forces of respective brakes in accordance with maximum frictional forces to be developed between the respective wheels and the road surface (hereinafter braking force proportioning function), in addition to anti-skid and traction control functions. The present invention also relates to an electronically controllable pressure modulator for use in a braking system having such functions.
2. Description of the Related Art
Frictional force between a vehicle wheel and a particular road surface is a function of a vertical load imposed upon the wheel perpendicular to the road surface, and of the coefficient of friction between the wheel and the road surface. The load on the wheel, in turn, varies in response to a weight transfer and position of the vehicle. For example, when the vehicle makes a turn, higher loads are imposed on the outer wheels, and thus the inner wheels are subjected to lower loads. Also, during hard braking, the weight of the vehicle is transferred to the front thereof, so that the front wheels are subjected to a higher load than the rear wheels. The vertical load is also dependent on the inclination of the surface on which the vehicle is travelling. The coefficient of friction between the wheels and the road, in turn, depends on the prevailing road surface conditions. It is known that the coefficient of friction is at a maximum when the slip ratio of the wheel is about 15 to 20 percent, as discussed, for example, in a copending U.S. patent application Ser. No. 877,729, filed June 24, 1986 and assigned to the assignee of the present invention.
If the braking force by a brake overcomes the maximum frictional force which can develop between the associated wheel and the road surface, the brake tends to lock the wheel and thereby cause wheel skidding. Also, when a driving torque transmitted to a driven wheel is higher than the maximum frictional force, the driven wheel spins and the wheel traction is decreased.
It is known in the art that a potential wheel skid or wheel spin condition can be eliminated by controlling the brake fluid pressure applied to brake wheel cylinders of skidding or spinning wheels. For example, Japanese Unexamined Patent Publication No. 56-90760, published July 23, 1981, discloses a braking system wherein, during an anti-skid operation, an electronic control unit senses the potential wheel skid condition and causes a modulator valve to close communication between the master cylinder and the brake to reduce the wheel cylinder pressure and restore the rotation of the skidding wheel. When the potential wheel skid condition has been eliminated, the control unit then causes the modulator valve to open communication between an accumulator and the brake to increase the wheel cylinder pressure and retard the rotation of the wheel. In this manner, the wheel cylinder pressure is repeatedly decremented and incremented in sequence at very short time intervals until the vehicle stops or until the emergency braking operation is no longer necessary. During a traction control operation, the control unit senses a potential wheel spin condition and causes the modulator valve to communicate the accumulator with the brake to retard the rotation of the wheel and restore wheel traction.
This braking system also comprises a pressure regulator which regulates the accumulator pressure in such a manner that, during the anti-skid operation, the brake fluid pressure is substantially equal to the fluid pressure generated by the master cylinder. During a traction control operation, the pressure regulator regulates the brake fluid pressure to be a fraction of the accumulator pressure.
Although this system adjusts the brake fluid pressure for acceptable performance during both anti-skid and traction control operations, it is unable to control the braking force of brakes prior to the occurrence of the potential wheel skid condition, since an anti-skid operation is initiated only when a potential wheel skid condition is sensed of at least one wheel. As in most other anti-skid systems, this braking system controls wheel skidding by successively decrementing and incrementing the brake fluid pressure only after the occurrence of the potential wheel skid condition, and no attempt is made to avoid the occurrence of the potential wheel skid condition. Moreover, this system requires a brake booster to assist the operator to apply the brakes during a normal braking or anti-skid operation, since the maximum fluid pressure communicated to the brake is equal to the master cylinder pressure.
Japanese Unexamined Patent Publication No. 48-22879, published Mar. 23, 1973, discloses a power brake device with an anti-skid function. This device includes a vacuum or pneumatic booster which amplifies the master cylinder pressure during a normal braking operation. The device further includes a solenoid operated control piston which reduces the brake fluid pressure during an anti-skid operation. With this device, all of the brake wheel cylinders are supplied with fluid at an equal pressure during the anti-skid operation, so that it is impossible to carry out a separate control of the braking force of each brake.
It is also known in the art to use a proportioning valve to limit fluid pressure to the rear brakes in order to avoid rear wheel skidding that can occur during hard braking because the weight of the vehicle is transferred to the front thereof. However, the proportioning valve is not responsive to road surface conditions or to weight transfer due to centrifugal force exerted when the vehicle turns.